An amplifier typically has low efficiency and large linearity margins at low-power regions, and high efficiency and small linearity margins at high-power regions. For linear amplifiers, the linearity is limited at the highest output power condition, which is known as the saturated region. The linearity and efficiency of an amplifier may be affected by the bias conditions of the amplifier.
Amplifiers may be classified depending on their associated bias level and current conduction angle. These classifications include class-A, class-B, class-AB, and class-C amplifiers. For instance, a class-A amplifier has the highest bias level with the highest linearity, and a class-C amplifier has the lowest bias level with the lowest linearity. In contrast, class-A amplifiers have the lowest efficiency, and class-C amplifier has the highest efficiency. This is typically because the efficiency of an amplifier has an opposite reaction to bias conditions than that of an amplifier's linearity.
Fundamental configurations of most conventional adaptive biasing schemes for power amplifiers are composed of a signal sampler, a low-pass filter, a power detector, and a bias feeding block. FIG. 1 shows a schematic diagram for a conventional power amplifier with a conventional adaptive bias circuit. It also shows signal spectrums and time-domain signals at several points assuming that the input signal is a two-tone signal. For the power amplifier (PA) shown in FIG. 1, an output signal is sampled by a signal sampler, and the sampled signal is filtered by a low-pass filter. The filtered signal power is detected by a power detector, and the detected signal is fed into the power amplifier through a bias feeding block. The bias of the power amplifier is dynamically changed depending on the output power of the power amplifier. Eventually, the adaptive biasing scheme adjusts the power amplifier to maximize efficiency with an allowable distortion.